触发器

触发器是一类特殊的事务，可以监视某种数据操作并触发相关操作

需求：
商品表：goods
订单表：rod
当下一个订单时，对应的商品要减少（买几个商品就少几个库存）


分析：
监视谁：ord
监视动作：insert
触发器：after
触发事件：update


create trigger t1
after
insert
on
ord
for each row
begin
update goods xxxx
end;


create table goods(
	gid int,
	name varchar(20),
	num smallint
)

create table ord(
	ord int,
	gid int,
	much smallint
)

insert into goods values
(1,'cat',34),
(2,'dog',65),
(3,'pig',21);

update goods set num=num-2 where gid=1;


触发器引用变量  
insert  新行  new
delete 	旧行  old


delimiter $
create trigger t1
after
insert
on ord
for each row
begin
update goods set num=num-2 where gid=1;
end$


show triggers;

买什么商品都只减少猫

create trigger t2
after
insert
on ord
for each row
begin
update goods set num=num-new.much where gid=new.gid;
end$


查看已有triggers:show triggers
删除已有trigger:drop trigger t1;


删除订单时的触发器
create trigger t3
after 
delete
on ord
for each row
begin 
update goods set num=num+old.much where gid=old.gid;
end$



清空表：truncate org;

insert into ord values(123,2,4)



修改订单时的触发器（仅限修改数量）

create trigger t4
before
update
on ord
for each row
begin 
update goods set num=num+old.much-new.much where gid=new.gid;
end$

思考：before与after之间的区别

思考：如果剩余3头猪，但是客户购买了10头猪，发生什么情况，能否预防
能否在购买量much>库存num时，把much自动改为num
before 关键字解决

在t2的基础上，完成much与num的判断

create trigger t5
after 
insert
on ord
for each row
begin
declare
rnum int;
select num into rnum from goods where gid=new.gid;
if new.much>rnum then
	set new.much=rnum;
end if;
update goods set num=num-new.much where gid=new.gid;
end$


updating of new row is not allowed in after trigger
原因：insert 之后，new行已经插入到表中，成为事实，改new已经晚了


create trigger t5
before
insert
on ord
for each row
begin
declare
rnum int;
select num into rnum from goods where gid=new.gid;
if new.much>rnum then
	set new.much=rnum;
end if;
update goods set num=num-new.much where gid=new.gid;
end$